1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a moldless semiconductor device, not covered with any mold resin. This invention also relates to a photovoltaic device module, a solar cell module and a construction material which make use of the moldless semiconductor device as a bypass diode.
2. Related Background Art
In recent years, it has been predicted that an increase in CO.sub.2 which causes the greenhouse effect will make the earth's environment warm, and thus there is an increasing demand for clean energy that releases no CO.sub.2. As an energy source that releases no CO.sub.2, atomic power generation can be considered, but the problem of radioactive waster remains unsettled. Thus, a clean energy having a higher safety is desired. Under such circumstances, among clean energy sources, solar cells attract notice especially greatly in view of cleanliness, safety and readiness to handle.
As types of solar cells, many types of solar cells such as crystal type solar cells, amorphous type solar cells and compound semiconductor solar cells are the subject of research and development. In particular, the amorphous type solar cells, though not comparable to the crystal type solar cells with respect to conversion efficiency, indeed, can be made large-area with ease and also have a large light absorption coefficient and still also can operate in thin-film construction, having excellent features the crystal type solar cells do not, and are one type of solar cells considered to have a promising future.
Now, usually, a one-sheet solar cell alone has not a sufficient output voltage when solar cells are viewed as electric power supply sources. Hence, it is necessary to use a plurality of solar cells connected in series or in parallel.
The most difficult point in the case where a plurality of solar cells thus connected in series are operated is that, when cells are partly shut out from sunlight because of being shadowed by a building or because of snowfall to come to generate no electricity, the total voltage generated from other devices which are normally generating electricity is applied directly thereto in the form of a reverse voltage. Then, when such a reverse voltage reaches a value higher than the breakdown voltage of a device, there is a possibility of the break of the device. Accordingly, in order to avoid such a break of the device, a diode must be connected in parallel to the device in the reverse direction for each device of the devices connected in series. Such a diode is commonly called a bypass diode.
As bypass diodes, mold-packaged diodes provided with cover resin around them have commonly been used as general-purpose articles.
FIG. 1 schematically illustrates a conventional mold-packaged semiconductor device. Reference numeral 301 denotes a semiconductor chip; and 302 and 303, outer-connecting terminals connected electrically with the semiconductor chip 301 through a soldering material (not shown). Reference numeral 304 denotes a mold resin such as epoxy resin, which protects the semiconductor chip mechanically and plays simultaneously a role of preventing moisture from entering. Thus, the mold-packaged semiconductor device refers to a device whose semiconductor chip 301 is covered completely with the mold resin 304 and in which neither the semiconductor chip nor the outer-connecting terminals at their part adjacent to the semiconductor chip are laid bare at all. Namely, the outer-connecting terminals are seen from the outside only in part.
However, when such mold-packaged diodes are used as the bypass diodes of solar cells, there are the following difficult points.
(1) While a solar cell itself has a thickness of about 300 .mu.m, even a mold-packaged diode having a small thickness has a thickness of about 1 mm. As the result, the solar cell has an extremely large thickness only at the part where the diode is provided, so that the flatness of a module is greatly damaged. To keep the flatness, a method may be used in which a covering material around the cell is made thick, which, however, results in a high cost for the covering material. PA0 (2) Since the diode chip itself is covered completely with the mold resin, electric currents flowing through the diode chip may accelerate heat deterioration of the covering material around it because the diode has a very poor heat dissipation against the heat generated at the part of p-n junction of the chip. In an instance where a solar cell has a metal substrate as a component, the metal may be used as a fin. However, in the case of the mold-packaged diode, it is impossible for the chip portion to be brought into contact with the metal substrate because the chip is always covered with the mold resin, and there is also a limit to the improvement in heat dissipation. PA0 (1) Because of the absence of a mold resin, the device is very fragile to torsional or bending force. Stated specifically, when the moldless diode is connected by soldering to a photovoltaic device, the outer-connecting terminals bend upon application of the torsional or bending force, and a shear stress is applied to the chip. As the result, the chip may have a residual stress and, in a serious instance, may break, where the reliability required as diodes is greatly damaged. PA0 (2) Because of the absence of a mold resin, the device is very fragile to external force, in particular, to an impact force. As the result, the chip may break like the instance of (1). PA0 (3) In a state where the moldless diode is connected to a photovoltaic device module or in a condition where the photovoltaic device module is installed on the roof surface of a house, the outdoor weather such as wind and hail may affect the photovoltaic device module itself to exert bend and impact repeatedly thereon. As the result, similar force may act also on the diode connected in the interior to cause the diode to break in some cases.
Such a background has brought a proposal of constitution where the chip diode is used without any mold-packaging resin (hereinafter "moldless diode") as disclosed in, e.g., Japanese Patent Applications Laid-Open No. 5-291602 and No. 9-82865. In the case of the moldless diode, the flatness of the module can be kept because of a very small thickness of about 300 .mu.m, and also the chip portion can come into contact with the metal substrate because there is no mold resin. Thus the heat dissipation can be improved.
FIGS. 2A and 2B schematically illustrate an example of such a moldless diode. FIG. 2A is a plan view, and FIG. 2B a cross-section along the line 2B--2B. In these drawings, reference numeral 201 denotes a semiconductor bare chip; 202 and 203, outer connecting terminals. The chip is connected electrically with the connecting terminals through a soldering material (not shown).
In such a conventional moldless semiconductor device, however, the chip portion is not covered completely, and hence it is very difficult to handle it, bringing about problems as discussed below.